1. Field of the Invention
The present invention relates to a zeolite honeycomb structure. More particularly, it relates to a zeolite honeycomb structure having a low pressure loss and an excellent NOx purification performance.
2. Description of the Related Art
It is known that zeolite is a type of silicate having a mesh crystal structure provided with fine pores having a uniform diameter, there are present various chemical compositions represented by the general formula: WmZnO2n.sH2O (W: sodium, potassium, calcium or the like, Z: silicon, aluminum or the like, and s takes various values), and there are present many kinds (types) of crystal structures having different pore shapes. Zeolite has an inherent adsorption ability, catalyst performance, solid acid characteristics, ion exchange ability and the like based on the respective chemical compositions or crystal structures, and is utilized in various use applications such as an adsorbing material, a catalyst, a catalyst carrier, a gas separation film, and an ion exchanger.
For example, MFI-type zeolite (also referred to as “ZSM-5 type zeolite”) is zeolite provided with pores each having a size of about 0.5 nm by oxygen ten-membered rings among crystals, and is utilized in a use application such as an adsorbing material which adsorbs nitrogen oxides (NOx), hydrocarbons (HC) or the like from a car exhaust gas, or a gas separation film which selectively separates only p-xylene from a xylene isomer. Moreover, Deca-Dodecasil 3R (DDR) type zeolite is zeolite provided with pores of about 0.44×0.36 nm by oxygen eight-membered rings among crystals, and is utilized in a use application such as a gas separation film which selectively separates/removes only carbon dioxides from a natural gas or a biological gas to improve purity of methane which is useful as a fuel.
For example, there is suggested a selective reduction type catalyst including a catalyst which removes nitrogen oxides contained in an exhaust gas discharged from an engine for a car, an engine for a construction machine, an industrial stational engine, a burning apparatus or the like. Specifically, on the surface of an integral structure type carrier (C), there are disposed a lower catalyst layer (A) having a function of oxidizing nitrogen monoxides (NO) in the exhaust gas, and an upper catalyst layer (B) having a function of adsorbing ammonia. The lower catalyst layer (A) contains a noble metal, and the upper catalyst layer (B) includes zeolite (e.g., see Patent Document 1).
Moreover, there is suggested a catalyst including zeolite particles onto which 0.01 to 0.2 wt % of a noble metal is loaded and zeolite particles onto which 3 to 15 wt % of copper is loaded as main constituent particles, and particle diameters of both types of the particles are in a range of 1 to 20 μm (e.g., Patent Document 2).
In selective catalytic reduction (hereinafter abbreviated as “SCR” sometimes), for example, ammonia (NH3) is used as a reducer which decomposes NOx, and in the presence of ammonia, NOx is decomposed by a catalyst containing titanium oxide, vanadium oxide, zeolite and the like as main components. In such selective catalytic reduction, NOx is finally reduced to N2 by reaction formulas (1) to (3) as follows.NO+NO2+2NH3→2N2+3H2O  (1);4NO+4NH3+O2→4N2+6H2O  (2); and6NO2+8NH3→7N2+12H2O  (3).
Reducing reaction of the above reaction formula (1) proceeds at a temperature of about 200° C. or lower, but reducing reaction of the above reaction formula (2) proceeds at a temperature of 200° C. or higher, and reducing reaction of the above reaction formula (3) proceeds at a temperature of 275° C. or higher. Therefore, for the purpose of improving purification performance, it is important to perform the reducing reaction of the above reduction formula (1) in which the reaction proceeds at the low temperature. In an exhaust gas discharged from a diesel engine in a typical operation state thereof, a ratio of NO2 in NOx is 0.5 or less. When the reducing reaction of the above reaction formula (1) is allowed to proceed, the amount of NO2 becomes inadequate.
For example, in the selective reduction type catalyst disclosed in Patent Document 1 described above, ammonia is adsorbed by zeolite of the upper catalyst layer (B), and utilized as the catalyst of the above reaction formula (1). Moreover, an excessive amount of NO is oxidized in the lower catalyst layer (A) to generate NO2, thereby allowing the reducing reaction of the above reaction formula (1) to proceed in the upper catalyst layer (B) again. Furthermore, when the amount of ammonia is excessively large, an excessive amount of ammonia is oxidized by the lower catalyst layer (A), whereby non-reacted ammonia can be prevented from being discharged to the environment.
Moreover, Patent Document 1 discloses a honeycomb structure in which the selective reduction type catalyst is not loaded onto the integral structure type carrier (e.g., a ceramic carrier made of cordierite or the like), but the honeycomb structure is constituted of a columnar honeycomb unit including inorganic particles containing an NOx adsorbing material and an ammonia adsorbing material and an inorganic binder, and including a plurality of cells partitioned by cell walls. The cell walls are coated with a zeolite containing material (e.g., see Patent Document 3).    [Patent Document 1] JP-A-2008-279334    [Patent Document 2] Japanese Patent No. 3285206    [Patent Document 3] WO2009/118868
However, a selective reduction type catalyst disclosed in Patent Document 1 is a catalyst loaded onto and used in an integral structure type carrier made of cordierite or the like, and this integral structure type carrier does not contribute to an NOx gas purifying function. The presence of the carrier causes a problem that a pressure loss increases.
Moreover, in the catalyst disclosed in Patent Document 2, there are mixed zeolite particles onto which a noble metal is loaded and zeolite particles onto which copper is loaded, and the respective types of particles have the same degree of size, which causes a problem that owing to the presence of the zeolite particles including the noble metal loaded onto the surface of the catalyst, ammonia necessary for purification of the NOx gas is oxidized before ammonia exerts a function of a reducer.
Furthermore, in a honeycomb structure disclosed in Patent Document 3, after forming the honeycomb structure by use of inorganic particles containing a noble metal, the surfaces of the cell walls are coated with zeolite again, which causes a problem that manufacturing steps become remarkably complicated. Moreover, it is remarkably difficult to regulate a film thickness of a zeolite film which coats the surfaces of the partition walls. If the zeolite film is excessively thin, ammonia necessary for the purification of the NOx gas is excessively oxidized in the same manner as in the catalyst disclosed in Patent Document 2. On the other hand, if the zeolite film is excessively thick, the problem of the increase of the pressure loss occurs.